def prost(num):
    for i in range(2, int(num**0.5) + 1 ):
        if num % i == 0:
         return True  
def max_prost(num):
        if num < 1000 or num > 9999:
            return 0
        t4 = num // 1000
        t3 = (num % 100) // 100
        t2 = (num % 100) // 10
        t1 = num % 10
        maxim = 0
        for cur in (t1, t2 , t3, t4, t4*10 +t3, t3*10 +t2, t2*10 +t1, t4*100 +t3*10+t2, t3*100 +t2*10+t1, num):
            if prost(cur) and cur>maxim:
                maxim= cur
                return maxim
num = int(input())
res = max_prost(num)
print(res)



